Rock star ____ John (5)
I believe the answer is:
elton
'rock star john' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for elton that I've seen before include "..... Jonn (musician)" , "Ben --; -- John" , "Sir - - John, singer songwriter" , "One John" , "Let on he's John, the singer" .)